Addendum
• When WRED is configured on the global service-pool (regardless of whether ECN on global service-
pool is configured), and one or more queues have WRED enabled and ECN disabled, WRED is
effective for the minimum of the thresholds between the queue thresholed and the service pool
threshold.
• When WRED is configured on the global service-pool (regardless of whether ECN on global service-
pool is configured), and one or more queues are enabled with both WRED and ECN, ECN marking
takes effect. The packets are ECN marked up to shared- buffer limits as determined by the shared-
ratio for that global service-pool.
WRED/ECN configurations for the queues that belong to backplane ports are common to all the
backplane ports and cannot be specified separately for each backplane port granularity. This behavior
occurs to prevent system-level complexities in enabling this support for backplane ports. Also,
WRED/ECN is not supported for multicast packets.
The following table describes the WRED and ECN operations that occur for various scenarios of WRED
and ECN configuration on the queue and service pool. ( X denotes not-applicable in the table, 1 indicates
that the setting is enabled, 0 represents a disabled setting. )
Table 5. Scenarios of WRED and ECN Configuration
Queue
Configuration
Service-Pool
Configuration
WRED Threshold
Relationship
Q threshold = Q-T,
Service pool
threshold = SP-T
Expected Functionality
WRED ECN WRED ECN
0 0 X X X WRED/ECN not applicable
1 0 0 X X Queue based WRED,
No ECN marking
1 X Q-T < SP-T
SP-T < Q-T SP based WRED,
No ECN marking
1 1 0 X X Queue based ECN marking above queue
threshold.
ECN marking up to shared buffer limits of the
service-pool and then packets are tail dropped.
1 X Q-T < SP-T
SP-T < Q-T Same as above but ECN marking starts above
SP-T.
Configuring WRED and ECN Attributes
The mechanism to configure a weight factor for WRED and ECN functionality for backplane ports is
supported on the S6000 platform. However, this mechanism to configure a weight for WRED and ECN
functionality for front-end ports is supported on the S6000 and Z9000 platforms. A global buffer pool
that is a shared buffer pool accessed by multiple queues when the minimum guaranteed buffers for the
queue are consumed can be configured on the S6000 and Z9000 platforms.
WRED drops packets when the average queue length exceeds the configured threshold value to signify
congestion. Explicit Congestion Notification (ECN) is a capability that enhances WRED by marking the
packets instead of causing WRED to drop them when the threshold value is exceeded. If you configure
ECN for WRED, devices employ this functionality of ECN to mark the packets and reduce the rate of
sending packets in a congested, heavily-loaded network.
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Quality of Service (QoS)